A capacitive micromachined ultrasonic transducer (CMUT) is a device that is capable of sensing and/or generating acoustic energy. In a CMUT, a membrane layer is present that can be mechanically coupled to the medium of interest (and can therefore act as an acoustic transducer), and which is one electrode of an electrical capacitor. Acoustic deformation of the membrane alters the electrical capacitance, thereby providing an acoustic sensing capability. Conversely, an applied electric voltage on the capacitor can alter the position of the membrane, thereby providing an acoustic generation capability. It is often desirable to provide a large array of CMUT devices in practice. For example, applications such as medical imaging frequently require large CMUT arrays.
Two basic approaches are known for making CMUT devices and CMUT arrays. The first approach can be referred to as wafer bonding, and includes a wafer bonding step where a wafer containing the CMUT membrane layer is bonded to a second wafer to form the complete CMUT devices. US 2006/0075818 is a representative example of this approach.
The second approach can be referred to as sacrificial release fabrication, where a sequence of processing steps all applied to the same wafer is employed to form the CMUT membrane layer and to release it from surrounding material. US 2005/0177045 is a representative example of this approach.
Thus far, monolithic integration of CMUTs with integrated circuits has only been demonstrated with the sacrificial release CMUT fabrication approach as opposed to the wafer bonding CMUT fabrication approach. The reason for this is that integrated circuits cannot survive the high temperatures of CMUT wafer bonding. The example of US 2006/0075818 describes a CMUT wafer bonding process that includes a 2 hour anneal at 1100° C., which would destroy any conventional integrated circuitry present on the wafers being bonded.